Michael Muskal and Tina SusmanThis post has been corrected. See note below.

NEW YORK – To the solemn lament of tolling bells and the mournful recitation of names of the fallen, Americans on Wednesday paused to commemorate the moment of the worst terrorist attack on U.S. soil.

Memorial events unfolded at the site of the former World Trade Center, the White House, the Pentagon and near Shanksville, Pa., as the nation reflected on the 12th anniversary of the attacks by airliners commandeered by Al Qaeda that killed nearly 3,000 people.

As in past years, the ceremony at the site of the former Twin Towers began with a moment of silence and the ringing of a bell to mark the instant when the first jet hit the North Tower at 8:46 a.m. local time. Relatives of those killed then began the slow reading of names, a somber process punctuated by moments of silence and bell-ringing to mark the crashes of jets into the South Tower, the Pentagon and a rural field in Pennsylvania, as well as the collapse of each Trade Center tower.

Promptly at 8:46 a.m. in Washington, President Obama, joined by the First Lady Michelle Obama, Vice President Joe Biden and his wife, Jill, along with members of the White House staff, walked out to the South Lawn. The president later went to the Pentagon, where a jetliner struck at 9:37 a.m.

“We pray for the memory of all those taken from us -- nearly 3,000 innocent souls,” Obama said at the Pentagon.

“Our hearts still ache for the futures snatched away, the lives that might have been -- the parents who would have known the joy of being grandparents, the fathers and mothers who would have known the pride of a child’s graduation, the sons and daughters who would have grown, maybe married and been blessed with children of their own," the president said. "Those beautiful boys and girls just beginning to find their way who today would have been teenagers and young men and women looking ahead, imagining the mark they’d make on the world."

Among those gathered at the Pentagon were family members of those killed on Sept. 11, 2001. Many wore red, white, and blue ribbons and some cried as the president spoke.

The president also paid tribute to the four Americans killed a year ago in an attack on a U.S. compound in Benghazi, Libya, asking the country to pray for those who “serve in dangerous posts” even after more than a decade of war.

Obama spoke hours after he addressed the nation about the crisis in Syria and defended his policy calling for limited military strikes while also using diplomatic efforts to strip control of chemical weapons from the ruling regime.

“Let us have the wisdom to know that while force is at times necessary, force alone cannot build the world we seek,” Obama said at the Pentagon. He later added, “Let us have the confidence in the values that make us American, which we must never lose, the shining liberties that make us a beacon of the world; the rich diversity that makes us stronger, the unity and commitment to one another that we sustain on this National Day of Service and Remembrance.”

The National Day of Service and Remembrance was started in 2002 as a way to honor the spirit of cooperation that followed the aftermath of the deadly attacks. In 2009, Congress made it an official national day.

Members of Congress also marked the day by gathering outdoors in a solemn ceremony.

Unlike some past remembrances there were no speeches at the 2-year-old Memorial Plaza in lower Manhattan. Commuters, going about their usual business, walked around the former scene of devastation even as the ceremonies took place.

In lieu of speeches in New York, there was the occasional personal touch. “As time passes and our family grows, our children remind us of you,” Angilic Casalduc said of her mother, Vivian Casalduc. “We miss you.”

In New York, the ceremony came at a pivotal moment: A day earlier, voters began choosing nominees for the next mayoral election, their selections reflecting the challenges of securing a city still seen as a high-profile target of terrorists.

Joseph Lhota, a Republican closely associated with Rudolph Giuliani, who was mayor at the time of the attacks, easily won his party’s mayoral nomination Tuesday. But Democrats indicated a desire to move on from the strict policing practices of Police Commissioner Raymond Kelly, which have included spying on Muslims. The top vote getter in the Democratic primary was Bill de Blasio, who has vowed to replace Kelly.

Giuliani has warned that if policing is altered it will open New York up to another terrorist attack. Kelly and the outgoing mayor, Michael Bloomberg, say the city has been the target of more than a dozen foiled attacks since Sept. 11, 2001.

Bloomberg was among the dignitaries at the site, along with George Pataki, who was governor during the attack. Also present was New Jersey Gov. Chris Christie.

In Shanksville, Pa., the third scene of the attack, families of the passengers and crew aboard United Flight 93 gathered and remembered their loved ones as heroes. It was on that flight that passenger Todd Beamer famously issued the rallying cry “Let's roll,” as he and others rushed down the airliner's aisle to try to overwhelm the hijackers after learning of the coordinated attacks elsewhere. Instead of hitting a target, that flight crashed in the field.

“In a period of 22 minutes, our loved ones made history,” said Gordon Felt, the president of the Families of Flight 93, whose brother, Edward, was among the 33 passengers and seven crew members aboard the flight traveling from Newark, N.J., to San Francisco when it was hijacked.

U.S. Interior Secretary Sally Jewell also recalled the sacrifice the passengers made. “We never know when we'll be called to lay down our lives for others,” she said at the ceremony.

[For the Record, 4:43 p.m. PDT Sept. 11: An earlier version of this post said the South Tower of the World Trade Center was hit first. Actually, it was the North Tower.]